This invention relates to a lash adjuster for automatically adjusting the valve clearance of a valve gear in an internal combustion engine.
In a line for feeding fuel to an internal combustion engine or a line for discharging exhaust gas, a valve gear is provided to open and close an intake valve or exhaust valve (hereinafter called simply valve) by the rotation of a cam. This valve gear includes a lash adjuster for automatically adjusting the valve clearance.
Such a valve gear includes a cam, a valve and a valve stem provided on the valve. When the end face of the valve stem is pressed against the end face of an adjuster screw by the force of a valve spring which presses the valve stem toward the cam, this force is transmitted to the cam through a lifter body to open and close the valve as the cam rotates. Generally, the lash adjuster is mounted between the cam and the valve stem provided on the valve.
Such lash adjusters are known in which a threaded hole having a closed end is formed in the lifter body, the adjuster screw in threaded engagement with the threaded hole is axially biased by an elastic member mounted in the threaded hole at its closed end, and the female threads of the threaded hole and the male threads of the adjuster screw are serration-shaped such that the flank angle of the pressure flanks, which receive the push-in load applied to the adjuster screw, is greater than the flank angle of the clearance flanks to adjust any valve clearance. Such adjusters are disclosed in U.S. Pat. No. 4,548,168, and JP patent publications 11-324617 and 11-324618.
In such a lash adjuster, when a valve clearance tends to develop between the valve stem and the adjuster screw due, for example, to thermal expansion of the cylinder head, the adjuster screw moves axially while rotating along the clearance flanks under the push-in force of the elastic member, thereby absorbing the valve clearance. Conversely, when the adjuster screw is acted upon by a push-in force from the valve stem, it retracts until an axial gap formed at the thread engagement portions between the male and female threads disappears. When further push-in force is applied, it is borne by the pressure flanks, which are pressed against each other, thereby preventing the adjuster screw from retracting while rotating.
If the distance between the valve stem end and the camshaft shortens due, for example, to wear of the valve seat, the adjuster screw prevents the valve from being gradually pushed in due to axial variable loads applied from the camshaft, so that a pressure leak occurs because the valve is not completely shut even when the base circle of the cam abuts the cylinder head. At this time, the adjuster screw is further pushed in by an amount corresponding to the play of the threads from a position where the minimum value of the axial variable loads is zero, but never retracts any further.
Serration-shaped threads used for such a lash adjuster have two kinds of flanks, i.e. pressure flanks, which receive push-in loads applied to the adjuster screw, and clearance flanks, and have self-sustainable friction coefficients μs determined univocally by the friction coefficients μ between the thread surfaces of the male threads and female threads on the respective flank surfaces, and thread specifications. Generally, the threads are designed such that the self-sustainable friction coefficient μs of the pressure flanks is smaller than the friction coefficient μ between the thread surfaces, and that the self-sustainable friction coefficient μs of the clearance flanks is greater than the friction coefficient μ between the thread surfaces.
Specifically, the friction coefficient μ between the thread surfaces in such a lash adjuster is experimentally known to be about 0.1–0.15. For example, in the embodiments of the inventions described in the above-mentioned three patent publications, by setting the lead angle α=11.5°, pressure flank angle θ1=75°, clearance flank angle θ2=15°, the threads can be designed such that the self-sustainable friction coefficient μs of the pressure flanks is smaller than the friction coefficient μ between the thread surfaces, and the self-sustainable friction coefficient μs of the clearance flanks is greater than the friction coefficient μ between the thread surfaces (see FIG. 8).
On the other hand, in recent years, in automotive engines, for the purpose of reducing friction and direct contact of slide portions, motor oil containing organic molybdenum (friction modifier oil; hereinafter referred to as FM oil) is generally used. By using FM oil, a film that has an extremely low friction coefficient is formed on slide portions, so that slide resistance of various portions decreases. This helps to improve the fuel cost of automobiles. Typical organic molybdenums include molybdenum dialkyldithiocarbamate sulfide (alias molybdenum dithiocarbamate; MoDTC), and oxymolybdenum sulfide.dialkyldithiophosphate (alias molybdenum dithiophosphate; MoDTP). They have friction relaxing property, wear resistance, extreme pressure property, and oxidation resistance.
These effects are achieved in cooperation with ZnDTP (zinc dialkyldithiophosphate) which is an oil additive, and it is known that the friction coefficient can be reduced more markedly than if used alone. It is said that this is because ZnDTP forms iron phosphate on the substrate, and forms an MoS2 film thereon. Also, ZnDTP is high in reactivity with iron, and it is reported that such a tribochemical reactive film is not formed on slide surfaces provided with, for example, DLC film due to its chemical stability (technical magazine “TRIBOLOGIST” Vol. 47/No. 11/2002/page 819).
But in an engine in which is mounted such a lash adjuster, if FM oil described above is used, the friction coefficient μ between the thread surfaces may drop extremely to about 0.04. If the friction coefficient μ falls below the self-sustainable friction coefficient μs of the pressure flanks, slip may occur on the pressure flanks. If slip on the pressure flanks is excessive, when axial load is applied to the lash adjuster, the adjuster screw is pushed in, thus causing valve lift loss and causing the valve to get impulsively seated, thus producing abnormal sounds.
An object of this invention is to provide an improved lash adjuster for a valve gear employing a serration-shaped thread mechanism which suppresses the formation of tribochemical reactive film by using materials for the adjuster screw and the nut member, or for the thread surfaces thereof, with which the friction coefficient between the thread surfaces will not extremely decrease even under conditions in which FM oil is used for the engine.